Add support for PHP to beforeSendTransaction (#5864)

Author: cleptric

src/platform-includes/configuration/before-send-transaction/php.laravel.mdx

@@ -0,0 +1,13 @@
+In the Laravel config, a closure can be used to modify the event or return a completely new one. If you return `null`, the event will be discarded.
+
+```php {filename:config/sentry.php}
+'before_send_transaction' => function (\Sentry\Event $transaction): ?\Sentry\Event {
+    return $transaction;
+},
+```
+
+<Note>
+
+Learn more in [Closures and Config Caching](/platforms/php/guides/laravel/configuration/laravel-options/#closures-and-config-caching).
+
+</Note>

src/platform-includes/configuration/before-send-transaction/php.mdx

@@ -0,0 +1,10 @@
+In PHP, a closure can be used to modify the event or return a completely new one. If you return `null`, the event will be discarded.
+
+```php
+\Sentry\init([
+    'dsn' => '___PUBLIC_DSN___',
+    'before_send_transaction' => function (\Sentry\Event $transaction): ?\Sentry\Event {
+        return $transaction;
+    },
+]);
+```

src/platforms/common/configuration/options.mdx

@@ -567,7 +567,7 @@ This function is called with an SDK-specific message or error event object, and
 
 </ConfigKey>
 
-<ConfigKey name="before-send-transaction" supported={["javascript", "node"]}>
+<ConfigKey name="before-send-transaction" supported={["javascript", "node", "php"]}>
 
 This function is called with an SDK-specific transaction event object, and can return a modified transaction event object, or `null` to skip reporting the event. One way this might be used is for manual PII stripping before sending.